The Effects of Magnetic Resonance Imaging (MRI) On Tensile Strength and Indentation Hardness of Acrylic Resin Denture Base Materials

Aims: This study aims to evaluate the effect of (MRI) on physical, and mechanical properties of acrylic resin denture base materials. Materials and Methods : one hundred and ten samples were the total no. of samples, samples were divided into two groups each group contain two fifty five, the 1st sample group pink and 2nd groups clear heat cured acrylic resin, then the two groups is divided into four sub-groups, follow that the samples were exposed to magnetic resonance image at three different peri-ods of time (5, 15, 30) minutes within control group respectively and samples tested for indentation hardness, tensile strength. Results : the results obtained to show that there was a change in the physical properties “heat cured” acrylic resin weather it is “Pink” or “Clear” after exposure to (MRI) and this change happened at different levels and variable degree, also it has shown that there was a slight tendency to change order of a arrangement of atoms within each molecules with no well and clear evidence of altering of the main material itself, at least at circumstances of experiment. Conclusion : Exposure to (MRI) at different periods of time lead to altering of physical properties and at different levels of significant.


INTRODUCTION
Magnetic resonance imaging (MRI) is a non invasive medical test that helps physicians in diagnosis and treat medical condition and provide detailed pictures of organs soft tissue, bone and other body structure (1) .
The hydrogen nuclei in the patient and protons behave like tiny moments since hydrogen makes up 80% of all atoms found in the human body and exhibiting a "magnetic moment", under normal circumstances and each has a north and south pole (2) .
Research carried out date suggests that there are no adverse eff ects caused by short term exposure of head to static magnetic fi eld up to "2" tesla (T) (each T-10000 of earth gravity) whole body exposure should not exceed 4T, since most of medical research not exceed "2T" (3) .
Magnetic biological research deals with interaction of biological systems with neck static and/or on frequency ultra-low intensity electromagnetic fi elds with and/or via possible phenom-ena (4) . • crystallization of iron-bearing mag-netic mono-particles in tissue of or-ganism. • Possible existence of long-lived rotational states of some molecules inside protein parts. • Bending and releasing electrons up to new levels of energy states Chemical changes measured via (NMR) and it is due to the electronic environment around nuclei, since mag-netic fi eld are not equal electron (5) .
Iron atoms are ferromagnetic material, which are easily to lined up under the action of external magnetic fi eld due to strong inter action between atoms which was called "exchange forces" (6) .
(MRI) scanners is very useful for defi ne identical changes in material in three orthogonal planes with no risk of any interaction in case of "human-material" experiment (7) .
Aims This study aims to evaluate the eff ect of (MRI) on physical, and mechanical properties of acrylic resin denture base materials.

Data Collection:
In this study, we used two types of acrylic resin "Pink, Clear" both are heat cured acrylic resin, both types are important denture base material. The total number of specimens were one hundred and ten, the samples were di-vided into two groups each group con-tain two hundred twenty seven : 1. 1st. group samples were "Pink" heat cured acrylic resin. 2. 2nd. Group samples were "Clear" heat cured acrylic resin. Then, the total no. of samples of these two main groups were divided into four sub-groups, each sample were cured according to ADA specifi cation '' No. 13,2009'', samples were cured for 90 minutes in 70℃ follow by 30 minute in 100℃ using thermostatically controlled curing unit, after that the samples have incu-bated into distilled water at 37±1℃ for two days for conditioning before testing (ADA specifi cation), follow that sam-ples were exposed to magnetic resonance image (MRI) at three diff erent time periods (5, 15, 30) minutes respec-tively, and at end of each period of time, samples were tested for ''Indenta-tion hardness, tensile strength'' The samples preparation started by preparing a mold by investing a hard elastic foil then water and stone mixed in a ratio of 100:23 "powder-water", with manual spatulation for (10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) second, mixture placed in fl ask, the two halves were closed after good vi-bration , left for one hour for complete setting, wax elimination was done by immersing the metal fl ask in boiling water for (4) minutes (8) .
Each specimen after complete fi nishing and polishing well placed in-side distilled water for condition before testingthen, samples placed inside (MRI) device'' Philips'' (MRI) device and tested at diff erent periods of time (5, 15, 30) min.
Samples used for tensile strength procedure will be at dimension of '' 90*10*3±0.3'' mm. according to ADA specifi cation No.12 as seen in Figure (1), and the samples were tested by use of (Gunt universal testing machine) as seen in Figures (2), (3) , as specimens were grasped by two arms of machine and pulling force will start at room temperature and result were recorded on computer, forces at failure was recorded in Newton (N) and by formula:  Statistical test used include one way analysis of vari-ance (ANOVA), Student's (t-test) and Descriptive statis-tics and Duncan's multiple range test for both tensile strength and for indentation hardness test.

RESULTS
Tensile strength means and standard deviation for the tested groups at diff erent period of time (5,15,30) min. respectively and control group for both "Pink, Clear" acrylic resin as shown in Figure (1). One way( ANO-VA) analysis of variance shown in Table (1) and Duncan's multiple range test as shown in Figure (1) for two types of acrylic demonstrated at P ≤ 0,05 a signifi cant diff erences at P ≤ 0,05 in ten-sile strength for both type of acrylic "Pink, Clear" at diff erent periods of time (5, 15, 30) minutes within control group.
since Figure (1-A) shown a higher means of tensile strength for (15) min. group while Figure (1-B) shown a higher means of (15 min.) group also than oth-er tested subgroups, paired sample T-test for both Pink and Clear acrylic comparing at four periods of time as in Table (2) show a signifi cant diff erences at four periods of time between Pink, Clear acrylic.  Rockwell indentation hardness means, standard deviation for the tested group at (5, 15, 30) minutes respectively and control for Pink and Clear shown in Figure (2). One way (ANOVA) analysis of variance Table  (3) and Duncan's multiple range at P≤0.05 shown signifi cant diff erences for both types of acrylic "Pink, Clear" Figure (2-A) show a high means for 30 min sub-group of "Pink" acrylic , while Figure (2-B) shown a higher means for (15) minutes group than other tested groups.
Paired sample T-test for both Pink and Clear acrylic group comparing means of hardness at four diff erent pe-riods of time as in Table (4) a signifi cant diff erences in hardness mean at four period.

DISCUSSION
The purpose of this study to evalu-ate the diff erences that expected to be happened in acrylic resin denture base material in patients who suspected to visit (MRI) unit diagnosis for cancer or other oro-facial problem and study the possible physical changes that might be happened on material due to exposure to (MRI).
In this study, tensile strength test usually investigate the ,ultimate strength and the resistance of material to catastrophic failure (11,12) , it has been found that increase the tensile strength and mechanical properties of heat cured denture base acrylic is improved by the use of new (dental polymer) (13) to in-crease dimensional stability and fi xation against "MRI" wave to change in poles direction and reduce magnetic eff ect (14) , and results shown and obtain in Figure (1) explained by existence of ferromag-netic substances increase magnetization for (Pink) acrylic and for (Clear) acrylic explain by contain of more spaces in between polymer chain and reduction of magnetic fold to about million fold than applied fi eld at low time exposure and high time exposure (15) , and results shown in Table (2) depend and explain basically by diff erent composi-tion of two types of acrylic (Pink, Clear) (16) surface hardness (Rock Well) test is test of material resistance to in-dentation, and small size indentation the harder the material (17) . And the use of ((Scale L)) is for plastic and semi-plastic material for evaluation of hardness of rigid substance (18) although clinical use, den-ture base acrylic resin material im-mersed in saliva water, aqueous cleans-ing lead to increase in hardness, with high value for (Heat cured) acrylic res-in (19,20) and result obtain in Figure (2 A-B) agreed with conclusion as with the increase of magnetic resonance, the'' precession or lamor frequency'' is directly proportion-al to strength (21) and results shown in Table   Table (  (4) which show a signifi cant dif-ferences at four periods of time is ex-plain by Geometric distortion, which arise from Varity of factors, a part of it from chemical and susceptibility diff er-ences (22) CONCLUSION Exposure to (MRI) at diff erent peri-ods of time lead to change in physical properties of denture base material and this change happen at diff erent levels and degree in addition to change in the order of arrangement of atoms within same molecules which result in increasing vibration of bands in between two at-oms with out alter main atom and no change chemically has happened.